The fabrication of periodic inversely poled structures in bulk ferroelectrics was first developed over two decades ago. Since then, some main areas of research include the fabrication of inversely poled micro-domains in bulk ferroelectric crystals, and their possible applications for bulk acoustic wave ultrasonic transducers. Different bulk ferroelectric crystals (LiNbO3, LiTaO3, KTiOPO4, and BaTiO3) with one-dimensional periodic domain structures have been reported in a bulk media, but without specific applications for the acoustic devices, and without the embodiments of possible devices. The domain walls and associated complexes of crystal lattice defects also have been reported to be responsible for a new type of nonlinear ultrasonic attenuation in lithium niobate.
Recently, a domain resonance in two-dimensional periodically poled ferroelectric plates has been reported in Ostrovskii, A. B. Nadtochiy, Applied Physics Letters, 86, 222902, (2005) (hereinafter “Ostrovskii 2005 (1)”). In one prior art free periodically poled ferroelectric vibrator disclosed in Ostrovskii 2005 (1) and I. V. Ostrovskii, A. B. Nadtochiy, Free Vibration of Periodically Poled Ferroelectric Plate, Journal of Applied Physics, Vol. 99, p. 114106 (2006) (hereinafter “Ostrovskii 2006”), and shown in FIG. 1, the periodically poled multidomain free vibrator of the length L=2dN and thickness 2t consists of the differently polarized regions denoted as A1 and B1 named inversely polled ferroelectric domains. They have opposite directions of their axes of polarization shown as +z and −z. Mathematically this configuration is represented by opposite signs of piezoelectric coefficients e in the adjacent domains, that is +e for A-type domains and −e for B-type domains. This reflects a fact that the same type of mechanical strain generates opposite piezoelectric fields in A- and B-domains. The crystallographic reference frame is shown by the z- and y-axes that represent a piezoelectric plate of a standard ZY-cut chip. It has been shown that rf-admittance demonstrated a resonance-antiresonance behavior as disclosed in Ostrovskii 2005 (1); and I. V. Ostrovskii, A. B. Nadtochiy, Acousto-electric Characteristics of Periodically Poled Ferroelectric Plate, Condensed Matter, 0503/0503222 (2005) (hereinafter “Ostrovskii 2005 (2)”); and amplitude of free vibrations had maximum at certain frequency as taught by Ostrovskii 2006.
Ferroelectric chips are widely used for the fabrication of ultrasonic transducers and surface acoustic wave (SAW) devices for applications in ultrasonics, medicine, telecommunication, etc.
U.S. Pat. No. 4,464,639 discloses an interdigital transducer for Surface Acoustic Wave (SAW) excitation. In this patent and in U.S. Pat. No. 5,350,961, the inversely poled domains are mentioned in the sense that tiny interdigital electrodes are used to generate the SAW. This transducer must have the metal electrodes more thinly than an acoustic half-wavelength.
This requirement yields a technological restriction on upper limiting frequency since photolithographic technology can not make a very thin and still reliable electrodes needed for very high frequencies in the gigahertz frequency range. A problem is the metal interdigital electrodes must be too narrow (about 210 nm) for lithographic process and, in addition, these narrow metal electrodes are not stable over time as shown by Colin K. Campbell, “Understanding Surface Acoustic Wave (SAW) Devices for Mobile and Wireless Applications and Design Techniques,” (2004) (hereinafter “Campbell”). Another limiting issue is the nature of the surface acoustic waves propagating along crystal surface. In the case of very high frequencies of the gigahertz frequency range, a quality of mechanical polishing of a crystal surface becomes unsatisfactory, and high SAW attenuation does not permit one to fabricate a delay line. Industrial-scale SAW-device production based on optical lithography is feasible up to 2.5 GHz (2.5×109 Hz) (as shown by Hiroyuki Odagawa, and Kazuhiko Yamanouchi, SAW devices beyond 5 GHz., International Jour. High Speed Electronics and Systems, V. 10, N. 4, 111-1142 (2000) (hereinafter “Odagawa et al.”)), which is consistent with present global UMTS standard for 2 GHz communication in S-band of Super High Frequency (SHF). Due to commercial and security applications, it is anticipated that there will be an increase in operation frequency and subsequent shift to the range of 2 to 10 GHz range as shown by Campbell; Odagawa and S. Lethonen et al., “Surface acoustic wave impedance element filters for 5 GHz.”, Applied Phys. Letters, V. 75, N. 1. p. 142-144 (1999). Unfortunately, there is no current technology for very high frequency acousto-electric devices.
With regard to prior periodically poled ultrasonic transducers, the prior transducers consist of an array of inversely poled small single ceramic transducers that are brought together in one composite embodiment. FIGS. 2a and 2b show two ultrasound transducers disclosed in U.S. Pat. No. 5,360,007. In FIG. 2a, the transducers 12 are arranged to have alternately reverse directions of polarizations, and filling members 13 are provided between the transducers 12 so to prevent their lateral coupling. The electrodes 14, 15 are provided, and acoustic matching layers 16, 17 are formed on the ultrasonic wave transmission surface. A substrate 18 is necessary to hold this composite transducer. In FIG. 2b, the device has sectional electrode 14. In both cases, there is no acoustical coupling between the single piezoelectric elements 12 with inverse polarization. Substrate 18 is essential, and bulk acoustic waves are irradiated in the direction of polarization of the elements 12. Similar devices for composite ultrasonic transducers are often referred to as “ultrasonic transducer arrays” in the art.
High frequency or wide band ultrasonic transducers directed for use as bulk acoustic waves, consist of a thin piezoelectric film deposited on a substrate. For example, FIG. 3 depicts a multidomain surface acoustic wave transducer disclosed in Patent Application Publication No. US 2002/0135270. Unfortunately, the reference fails to teach or provide an enabling disclosure to allow one to make a multidomain surface acoustic wave transducer. The reference merely presents a theoretical idea about a multidomain film transducer consisting of the inversely poled domains D1 and D2, which are deposited on one or more substrates. Further, this reference does not describe an actual example of a device which could function in a manner speculated in the reference. Therefore, one skilled in the art cannot create such a surface acoustic wave transducer, as one would not know how to construct one.
All of the aforementioned devices and references relate to bulk or surface acoustic waves that are generated by some composite transducer. The current state-of-the-art in high frequency and buffer free ultrasonic composite transducers are limited by about 35 MHz. For example, for bulk acoustic waves, a film transducer consists of a very thin piezoelectric film deposited on a solid buffer. These transducers may be fabricated up to 1 GHz frequency, but it is actually a massive delay line; and, therefore, its size cannot be miniaturized at all. No prior device is able to be incorporated into a wide band ultrasonic transducer or ultrasonic chirp device.
There is a need in the art for new micro/nano technology designed for higher frequency acoustic devices, especially those designed for planar technology.